Nitride semiconductors having higher breakdown electric field and higher electron saturation velocities have been drawing attention as the next generation semiconductor materials for high-frequency and high-power devices. Particularly, because a multi-layered structure formed by laminating layers composed of AlGaN and GaN is characterized in generating a two dimensional electron gas (2DEG) having high concentration in a lamination interface (heterointerface) by large polarization effect (spontaneous polarization effect and piezo polarization effect) specific to nitride material, the development of a high electron mobility transistor (HEMT) using this multi-layered structure for a substrate has been actively made (for example, see “Highly Reliable 250 W High Electron Mobility Transistor Power Amplifier”, TOSHIHIDE KIKKAWA, Jpn. J. Appl. Phys. Vol. 44, No. 7A, (2005), pp. 4896-4901).
In order to commercialize such an HEMT device and a substrate for an HEMT device having a multi-layered structure used for manufacturing the HEMT device, various problems should be solved: problems relating to the improvement of performance such as power density increase and high efficiency, ones relating to the improvement of function such as a normally-off operation, fundamental ones such as high reliability and price cuts and the like. For each problem, active efforts have been made.
For instance, if the concentration of two dimensional electron gas inherent to the substrate for an HEMT device is greatly increased, controllable current density of the HEMT device, i.e. usable power density is increasingly improved. In the case of a nitride HEMT device having the most usual composition of forming a channel layer with GaN and forming a barrier layer with AlGaN, the two dimensional electron gas concentration has been known to increase in accordance with the increase of AlN mol fraction in AlGaN forming the barrier layer (for example, see “Gallium Nitride Based High Power Heterojunction Field Effect Transistors: process Development and Present Status at UCSB”, Stacia Keller, Yi-Feng Wu, Giacinta Parish, Naigian Ziang, Jane J. Xu, Bernd P. Keller, Steven P. DenBaars, and Umesh K, Mishra, IEEE Trans. Electron Devices 48, (2001), 552, and “Characterization of Different-Al-Content AlGaN/GaN Heterostructures and High-Electron-Mobility Transistors Grown on 100-mm-Diameter Sapphire Substrates by Metalorganic Vapor Phase Epitaxy”, Makoto Miyoshi et al., Jpn. J. Appl. Phys. Vol. 43, No. 12, (2004), pp. 7939-7943).
It has been attempted to individually control distortion stress and a energy band structure by forming the channel layer with GaN and forming the barrier layer with InAlGaN, thereby to improve electrical characteristics of the HEMT device (for example, see “Novel Quaternary AlInGaN/GaN Heterostructure Field Effect Transistors on Sapphire Substrate”, Yang Liu et al., Jpn. J. Appl. Phys. Vol. 45, No. 7, (2005), pp. 5728-5731).
It has been also attempted to form the channel layer with GaN, form the barrier layer with AlGaN, and form a capping layer with InAlGaN thereon, thereby to improve contact resistance characteristic of the HEMT device (for example, see Japanese Patent Application Laid-Open No. 2007-324263).
In the case of the HEMT device having a heterostructure of AlGaN/GaN disclosed in KIKKAWA, Keller et al., and Miyoshi et al., if a barrier layer is formed of AlGaN having a large AlN mol fraction in order to increase the two dimensional electron gas concentration, tensile stress generated inside the barrier layer is made larger. This deteriorates the quality of film and aggravates the surface morphology (e.g. increase of distortion, generation of cracks, and the like). As a result, various problems have been caused, such that high concentration of two dimensional electron gas is not obtained as expected, various contact characteristics such as ohmic characteristic, Schottky characteristic and the like is deteriorated and unnecessary surface level arises and causes deterioration of device dynamic characteristics.
It has been pursued to employ InAlGaN quaternary mixed crystal for the barrier layer as the HEMT device having the heterostructure of InAlGaN/GaN disclosed in Liu et al, thereby to improve the electric characteristics while individually controlling the tensile stress and the band structure produced inside the barrier layer. However, it is insufficient to be provided as a practical device because the mobility of the two dimensional electron gas is as low as 700 cm2/Vs due to the combined factor such that composition ratio of InAlGaN quaternary mixed crystal is inappropriate, and moreover, growth condition of InAlGaN quaternary mixed crystal is inappropriate, and so on.
The device disclosed in Japanese Patent Application Laid-Open No. 2007-324263 has the advantage of reducing the contact resistance and thus improving the device characteristics by forming the capping layer with InAlGaN, but the barrier layer is formed of AlGaN so that it has the same problem as KIKKAWA, Kellar et al. and Miyoshi et al. with respect to the two dimensional electron gas concentration.